Margaret Thatcher was urged by one of her closest Cabinet allies to run a campaign of fear to deter teenage girls from becoming pregnant, according to newly published files.

Education Secretary Sir Keith Joseph wanted the Government to produce a series of "scare" films in an attempt to curb the number of pregnancies among immature adolescents from "the least good homes".

Joseph - regarded as Thatcher's ideological mentor - believed a "sharply rising trend" of bad parenting was a "major cause of poor education and crime", and he had no doubt who was responsible, according to official papers released by the National Archives under the 30-year rule.

Prime Minister Margaret Thatcher and Sir Keith Joseph in 1980

"The young concerned tend to be the least mature from the least good homes. They embark on parenthood casually," he wrote in a memorandum to Thatcher, dated October 1982.

"Those girls who are at most risk will tend neither to restrain themselves nor insist on or use contraceptives nor to have sufficient grip even to consider abortion in sufficient time."

His solution, he acknowledged, would be controversial.

"One possibility - delicate and fraught with risk - would be to try to use, in connection with pregnancy, the approach used in connection with cigarette smoking - that is fear," he said.

Joseph - sometimes dubbed the Mad Monk - reminded Thatcher that when he was a minister at the Department of Health and Social Security he approved a series of four short films to try to scare people off smoking.

"They were widely noticed. Some of the most vulnerable may have been influenced because the films used hedonistic and short time-horizon arguments.

"We could therefore explore whether short scare-films, suggesting that maternity is marvellous when the parents concerned are ready for it, might be practicable."

His comments echo a controversial speech he made eight years earlier when he claimed that single mothers "who were first pregnant in adolescence in social classes 4 and 5" were threatening the balance of "our human stock".

His words, in 1974, raised the spectre of eugenics and were widely seen has having cost him the chance of the Tory leadership.